Clutches in which two clutch elements can disengage when the torque being transferred exceeds a predetermined value are known. Usually, one of the clutch halves or elements, or parts or components thereof, are shifted axially, or radially, against a spring force, which spring force determines the overload level of the clutch. Clutches of this type are used to protect machines and other devices and plants against overload. The torque being transferred is limited at a suitable position within the drive train of the machine or device, so that it cannot be overloaded and no damage can result. Inter-engaging overload clutches, that is, positive drive clutches which can disengage upon overload have a distinct advantage with respect to friction clutches. The friciotnal value of friction clutches is variable and difficult to determine. Positive engagement clutches, thus, can be set for comparatively determined and precise overload torque values, without relying on the uncertain frictional value with respect to maximum transmitted torque.
A typical engagement arrangement of the clutch halves utilizes facing teeth, or gear-like elements which, upon disengaging of the clutch, disengage from each other. Disengagement is effected by shifting at least one of the two clutch elements, or, by shifting the teeth or tooth gaps relatively to each other. Other types of positive engagement clutches, for example claw clutches are known and used.
After the clutch is disengaged due to an overload, the clutch usually remains in disengaged position until manually reengaged. Manual reengagement is effected by shifting the movable clutch element to again obtain interengagement of the clutch parts with respect to each other. The shift movement is usually axial.
Upon disengagement of one clutch element from the other, and resulting disengaged position of the clutch elements, the clutch elements can be reengaged precisely provided that the interengaging elements fit against each other, in other words, that teeth formed on one clutch element, for example, fit into tooth gaps of the other one.
In known overload clutches, facing teeth are usually pressed by axial force, usually spring force, against matching tooth gaps. The torque being transferred depends on the engagement angle of the teeth and the axial force, which tends to retain the teeth against each other, against which the torque acts to separate the teeth. As soon as the torque dependent axial force exceeds the predetermined maximum force, the axially shiftable clutch moves until the teeth come out of engagement; the clutch, thus, will spin, and hence disconnect torque drive.
Clutches of this type usually have essentially flat surfaces which are engaged with each other, the extent of inter-engagement of the surfaces depending on the torque to be transferred. The flat surface engagement, for example of adjacent flanks, is lost as the clutch tends to overrun, due to excessive torque. The reason therefor is that the axial movement of the switchable or movable clutch half rotates, as well as moves axially, determined by the angle of the flanks of engaged surfaces. Consequently, the associated flanks shift with respect to each other and surface engagement is lost. At a limiting position, when the clutch is about to disengage, the flanks are effectively in engagement only at the outer edges of the teeth, so that design engagement of flank surfaces is lost and changes to effectively only edge engagement. Upon disconnection of the clutch, which occurs, of course, under maximum loading, or overloading conditions, the engagement forces of these edges of the flanks lead to substantial wear and tear of the teeth, resulting in decrease of the lifetime of the clutch, particularly if overload conditions arise frequently. Furthermore, the end portions of the respective teeth tend to deform, reducing the reliability and adherence to design parameters of the clutch.
Clutches which are designed to transfer high torques require an arrangement which permits axial movement of one clutch half with respect to the other counter a resetting force. The space requirement for such clutches is substantial. To transfer high torques, the reset force must be substantial and, the already large clutch structure requires, additionally, further space for strong reset springs capable of applying the necessary torque transmitting and resetting force.
Clutches which contain many teeth and tooth gaps in inter-engagement with each other must be made very accurately; otherwise, and based on manufacturing tolerances, the loading carried by the respective teeth beocmes non-uniform which further detracts from clutch operation in accordance with design parameters. The size of the clutch structure frequently prevents locating the clutch at the most desirable positions; particularly in cases of high torque transfer it is desired to place the clutch as closely as possible to the apparatus which is to be protected against overload. A large clutch structure frequently cannot be accommodated close to its best and most desired location.